Various types of hydraulic dampers for use in automobiles or the like have been proposed, such as dampers capable of adjusting the damping force to match the condition of the road surface over which the automobile is running, in order to enhance the level of comfort and maneuverability of the automobile.
An example of this hydraulic damper of adjustable damping force type is disclosed in Japanese Utility Model Publication No. SHO-62-27726.
According to this hydraulic damper, the cylinder is divided into two chambers by a piston, and the differential pressure generated by movement of the piston in the cylinder by extension and contraction of the piston rod causes a first damping force generation valve to be opened or closed so as to generate the damping forces in the extension and contraction strokes.
While a bypass passage in communication with the two chambers is provided in the piston rod and is branched through a pipe into two passage routes internally and externally of the pipe, a second damping force generation valve is provided in the passage route on the exterior of the pipe so as to generate the damping forces in the extension and contraction strokes. This passage route is designed to be selectively opened or closed by a shutter which can be operated from the outside to allow the damping force to be changed.
In the other passage route running through the pipe, orifices disposed in two stages and in communication with the two cylinder chambers are provided, and a desired orifice may be selected by operating the shutter. A check valve is provided between the orifices in the two stages, and the check valve causes the orifice of the lower stage only to communicate with the two chambers in the extension stroke so as to control the flow passage area of this passage route.
Opening and closing of the first mentioned one passage route and control of the flow passage area of the other passage route allows the damping force to be adjusted to different levels.
There are, however, two problems with regard to the hydraulic dampers of the type described above, as summarized below.
Firstly, since the check valve is inserted in the shutter, assembly of the check valve is not easy, resulting in an inefficient assembly operation in the manufacturing process.
Further, since adjustment of the damping force is executed only through the orifice, this adjustment can only be executed in the low speed range of the piston rod operating speed, so that the damping force cannot be changed in the high speed range of the piston rod. Consequently, it is difficult to adequately adjust the damping force to match the condition of the road surface over which the vehicle is running.
Another type of conventional adjustable damping force type hydraulic damper is disclosed in Japanese Utility Model Public Disclosure No. SHO-61-190048.
In this hydraulic damper, a piston rod extends into the cylinder in coaxial relation with a piston slidably disposed in the cylinder and partitioning the interior of the cylinder into upper and lower chambers. On the end that is inserted into the cylinder is mounted a tubular member provided with a plurality of orifices which have different opening areas. A bypass passage is defined by a through bore formed in the piston rod and adapted to allow insertion of an operating rod for operating the shutter to selectively open and close the orifices. By operating this rod, the orifices can be selected as required to control the flow of the oil flowing through the bypass passage, thereby adjusting the damping force to be generated.
In the conventional hydraulic damper mentioned above, the bypass passage for adjustment of the damping force is defined between the inner circumference of the through bore of the piston rod and the outer circumference of the operating rod, resulting in the passage having a small area. Especially in a hydraulic damper with a piston rod having a relatively small diameter, it is impossible to make the area of the passage sufficiently large without sacrificing the strength of the piston rod to an unacceptable extent.
In these circumstances, the narrow passage causes the resistance to the oil flow to generate a large damping force. Hence, even if orifices are made relatively large in order to obtain a low damping force characteristic, the damping force cannot be suitable reduced because of the resistance of the passage, thereby making the desired adjustment of the damping force impossible. Especially in the case where a valve mechanism is provided in the bypass passage, it is desirable for the bypass passage to have a large area for oil flow in order that the valve mechanism may create no disturbance to the flow control.
The present invention has been proposed in consideration of the problems explained above with reference to the prior art and has as its object the provision of a hydraulic damper of the adjustable damping force type which offers a simpler assembly operation and is capable of adjusting damping forces not only in the low speed range of a piston rod but also in the high speed range thereof.
It is another object of the present invention to provide a hydraulic damper in which the oil passage for adjusting the damping force has a large area so that suitable adjustment of the damping force can be consistently accomplished.